1. Field of the Invention
The present invention relates to thin film structures for magnetic recording heads and more particularly, to laminated thin film structures formed of unique combinations of materials, the laminates having improved magnetic properties ideally suited for poles of read/write inductive heads.
2. Description of the Prior Art
Magnetic recording at very high areal density places stringent requirements on magnetic properties of the materials used for forming the poles of magnetic recording heads. Good reproducing sensitivity requires a pole material with a high initial permeability, .mu..sub.i, and a low coercivity, H.sub.c. Since ##EQU1## where M.sub.s is the magnetization at saturation a H.sub.k is the anisotropy magnetic field, a high .mu..sub.i suggests a choice of H.sub.k as small as possible. The most commonly used head material having a high .mu..sub.i and a low anisotropy field, H.sub.k is permalloy. However, it has been shown by Nakamura, et al., IEEE Trans. Mag. 21(5) 1985, that too low an anisotropy results in undesirable domain patterns in narrow poletips. For domain stability in poletips less than 5 .mu.m wide, it is desirable to use a material with H.sub.k greater than that achievable with permalloy. However, increasing H.sub.k to a high value has the adverse effect of decreasing the reproducing sensitivity by decreasing .mu..sub.i.
One prior art attempt to combine beneficial magnetic properties of two materials can be found in U.S. Pat. No. 3,639,699 to Tiemann, which discloses a thin film structure formed by a layer of magnetic material having a high initial permeability, such as permalloy and a second layer of magnetic material having a lower initial permeability and a higher saturation flux density than the permalloy layer. The low .mu..sub.i materials disclosed by Tiemann are pure cobalt, pure iron or a CoFe alloy. Tiemann teaches that the poletip of this prior art lamination is formed with the inner layer of permalloy and the outer layer of the lower permeability material. The aim of the Tiemann lamination is to provide a recording head having a read/write gap effectively wider for the writing function than it is for the reading function. Tiemann fails to recognize the effect of domains on recording efficiency and readback instability. The high moment materials suggested by Tiemann are likely to cause a multitude of metastable domain states in the permalloy, because these high moment materials have unacceptably high coercivity.
U.S. Pat. No. 3,867,368 to Lazzari includes a decoupling layer between the laminations of the high initial permeability material and the lower initial permeability material. As noted in Lazzari, the lower initial permeability materials, such as Co, Fe or CoFe, also have a higher anisotropy field than permalloy. Lazzari suggests that the exchange coupling between high and low permeability materials is undesirable and therefore inserts a layer that breaks down the coupling.
In the above described prior art, laminated thin film structures have been found to be deficient in that the materials used by Tiemann, namely Co, Fe and CoFe, have a high coercivity which acts to decrease the head stability and reproducing sensitivity. Addition of the non-magnetic spacer by Lazzari reduces problems caused by the high coercivity of one of the sublayers, but it also decreases potential benefits, when both magnetic layers have a low coercivity, and a relatively strong exchange coupling. Thus, there is a need to provide a thin film magnetic structure for use in read/write heads in which the reproducibility of the read operation is improved, without sacrificing head efficiency.